A Sound Pressure Level Meter Circuit is a gadget that helps us figure out how loud sounds are in a certain place.
It shows us sound levels visually, which is super helpful for different tasks like testing audio systems, checking noise pollution, and working in sound engineering.
This article talks about an easy way to build a sound pressure meter using a condenser microphone, an operational amplifier, and a full scale deflection FSD meter to measure sound levels accurately.
Circuit Working:
Parts List:
| Component | Specification | Quantity |
|---|---|---|
| Resistors | 10k, 100k, 100Ω | 1 each |
| 22k | 2 | |
| Capacitors | Electrolytic 1µF, 100µF, 220µF | 1 each |
| IC | CA3140 | 1 |
| Electret Mic | 1 | |
| ON/OFF Switch | 1 | |
| Bridge Rectifier 1N4007 Diodes | 4 | |
| Full Scale Deflection FSD Meter | 100µA | 1 |
| Battery | 9V | 1 |
The circuit gets its power from a 9V battery.
To turn the circuit ON and OFF there is a ON / OFF switch .
The electret mic picks up sound and changes it into electrical signals.
These signals go to the amplifier stage through the coupling capacitor C1 1µF.
The CA3140 operational amplifier IC1 is set up to make the weak signals from the electret mic stronger.
The resistors R1 and R2 10k and 22k along with capacitor C2 100µF help control how much the amplifier boosts the signal.
The feedback resistor R4 100k keeps everything stable and helps manage the relationship between the input and output of the amplifier.
The output from IC1 is then changed from AC to DC by the diode bridge D1, which gives a signal that matches the sound intensity.
Capacitor C3 220µF smooths out the DC signal.
This final DC signal goes to the FSD meter M1, which is set up to show the sound pressure level in microamperes (µA).
Formulas:
Below mentioned are the formulas for Sound Pressure Level Meter Circuit:
Amplifier Gain Formula
Gain = R4 / (R2 + R3)
where,
- R4 is the feedback resistor that sets the amplifiers gain
- R2 and R3 are input resistors.
This formula determines the voltage gain of the operational amplifier CA3140.
Calculations:
If R4 = 100k and (R2 + R3) = (22k + 22k ) = 44k
then,
Gain = 100k / 44k = 2.27
This means the input signal from the electret mic is amplified by a factor of 2.27
RC Time Constant Formula
τ = R × C
- R is the resistance
- C is the capacitance.
This formula calculates the time constant for an RC circuit.
Calculations:
C3 = 220µF and R5 = 100Ω
τ = 100 × 220 × 10⁻⁶ = 0.022 seconds.
The time constant determines how quickly the capacitor charges or discharges smoothing the rectified signal.
Meter Calibration Formula
Voltage ∝ Sound Pressure Level
The voltage coming out in DC is related to how loud the sound is that the microphone picks up.
The analog meter M1 is set up so that a certain voltage will show a specific movement on the meter.
For example, if the DC output is 1V the meter will move all the way to the top showing that the sound pressure level is at its highest.
How to Build:
To build a Sound Pressure Level Meter Circuit using IC CA3140 following are the steps to follow for assembling.
- Gather all the components as mentioned in the above circuit diagram.
- Connect pin 2 of IC1 CA3140 to one terminal of R5, and connect the other terminal of R5 to the positive of capacitor C3 and connect the negative of capacitor to GND.
- Connect pin 3 of IC1 CA3140 to junction of C1 capacitor and resistor R4, and connect the other end of resistor R4 to positive of capacitor C2 and connect the negative of capacitor C2 to GND.
- Connect the junction of resistor R4 and capacitor C2 to the junction of resistor R2 and resistor R3, connect the upper end of R2 with positive and the lower end of R3 with GND.
- Connect the upper end of resistor R1 to the positive supply and the lower end with capacitor C1 and positive supply of electret mic, and connect the negative of mic to GND.
- Connect pin 6 of IC1 CA3140 to one AC input of the D1 bridge rectifier.
- Connect the other input of D1 bridge rectifier with pin 2 of IC1.
- Connect the positive output of the D1 bridge rectifier with the positive of M1 meter, and connect the negative of D1 bridge rectifier to negative of M1 meter.
- Connect the positive of the 9V battery with the positive supply rail of the circuit through S1 switch, and connect the negative supply rail of 9V battery to GND of circuit.
Conclusion:
This easy sound pressure meter circuit is a great way to measure how loud sounds are.
It uses a condenser microphone and an operational amplifier to process the signals accurately.
Plus, the analog meter shows a clear reading of the sound levels.
This circuit is affordable and can be made with common parts, which makes it perfect for school projects and real life uses.